The invention relates to the field of detecting objects on a transparent glazing. In particular, the invention relates to a process and a device for detecting objects found on a windshield, comprising a detector unit connected to a data processing unit to view a section of the windshield from the inside, which data processing unit can control actuators in response to objects being determined on the windshield, whereby the detector unit is arranged at a certain distance behind the windshield.
DE 197 04 818 A1 discloses a device, also referred to as rain sensor, to detect raindrops on a windshield of a motor vehicle to control a windshield wiper motor. This prior art device works on the basis of a reflection principle, with total reflection occurring on the outside of the windshield if no objects are present on the outside of the windshield to interfere with this total reflection. If the light rays strike a raindrop, however, they are decoupled from the raindrop such that a portion is reflected toward a photodetector and the measured light intensity is reduced.
This prior art rain sensor uses a one- or two-dimensional CCD charged coupled device as the sensor array, with a focusing lens being connected ahead of the photosensitive surface. To analyze the structures detected by the sensor array, the transducers of the sensor array are read as a one- or two-dimensional value field from the sensor array and are compared with a reference value field. If sufficient correspondence with a reference value field is determined, a corresponding control signal is generated to control one or several actuators. The value field provided is a curve reproducing the intensities of a pixel series.
Although this rain sensor is suitable for detecting raindrops, it cannot be excluded that the information detected by the CCD is not also of a background-related origin. The resulting wiping action is therefore not always satisfactory. Furthermore, this device is not able to provide specific information with respect to other objects impeding visibility through the windshield. Even though various types of dirt on the windshield can also cause refraction and thus reduced total reflection of the light rays, this light intensity reduction detected by the photodetector cannot be evaluated in relation to the object.
Based on the discussed prior art, it is thus the objective of the invention to propose a process for detecting objects found on a windshield that can be used reliably to detect and differentiate objects that are found on the windshield.
A further objective of the invention is to provide a device of the generic class for detecting an object found on a windshield permitting largely interference-free detection and determination of different objects found on a windshield.
The process according to the invention for detecting objects found on a windshield uses the signal fluctuations of the entire image, or of a specific image area, present on the output side on the pixels of the sensor array when viewing a scene, which in the form of correspondingly transformed spatial frequency spectra can provide information on an object found on the windshield. With respect to the term xe2x80x9cspatial frequency spectrumxe2x80x9d or xe2x80x9cspectraxe2x80x9d as used in these explanations, it should be noted that this term also covers the characteristic of the spectral power density of a frequency band across the entire, or across a specific portion of, the sensor array. For this purpose, an aperture and lens are inserted in the beam path of the sensor array directed toward the windshield, such that the photosensitive surface of the sensor array is focused on the outside of the windshield and objects found on the windshield are thus sharply reproduced on the sensor array. The invention uses the fact that an optically unfocused and thus unclear image does not have sharp transitions from one object to the next. The component of high spatial frequencies, such as they occur, according to Fourier, in signal jumps with sharp transitions, is low in such an image segment. Within the focused image plane, however, sharp object transitions can be detected-provided objects are present. Within the context of these explanations, the term xe2x80x9cfocusedxe2x80x9d means a sufficiently sharp image of the object on the photosensitive surface of the sensor array such that this image is clearly distinguished with respect to its sharpness from objects of the environment. The use of a diaphragm connected ahead of the lens produces a certain depth of field range so that, for example, the inside surface of the windshield, compared to objects of the environment, is also in the focused range of the sensor array. If objects are present within the depth of field range of the device and thus on the outside of the windshield, the spectrum of the determined spatial frequency distributions in the upper frequency range considered will have greater values compared to an image segment represented by its spatial frequency spectra in which no objects adhere to the outside of the windshield.
The process or the device, according to the invention, provides information regarding the objects adhering to a windshield based on the different image properties of different objects with respect to their spatial frequency spectra. Raindrops, for example, can be differentiated from opaque objects in that they have a dark margin along the edge while the interior of a raindrop is light. Furthermore, the fact can be used that a raindrop itself can act as a lens so that the environment in front of the windshield appears sharp and reversed on the sensor array. Due to this reversed image, the lower part of a raindrop is light (=sky) and the upper part is dark or darker (=environment). All these characteristics are expressed in intensity fluctuations in the high-frequency range of the spatial frequency spectra. Accordingly, the number of objects adhering to a windshield is determined across the entire image segment being considered. Making use of this fact, an exemplary embodiment provides that a vertically oriented sensor line be used as the sensor array.
According to a first proposed solution, once the spatial frequency spectra of an image segment have been determined, they are compared with reference frequency spectra that are suitably stored in a memory module. If there is sufficient correspondence between the detected spatial frequency spectra and a reference frequency spectrum for example, when a certain raindrop density is determined-a control signal is generated, which is used to actuate a specific actuator, e.g., a windshield wiper motor to wipe away the raindrops.
The distinction between different objects can be used to control not only the windshield wiper motor, but if dust or mud splatters are detected, also the windshield washer to remove the dirt.
According to a second proposed solution, the spatial frequency spectra of the pixels of the sensor array combined into blocks are determined as the characteristic of the spectral power density of one or several frequency bands across all blocks and are subsequently evaluated by neural analysis. This analysis is carried out in several steps. In a first analysis step, the determined spatial frequency spectra of the individual blocks are converted by means of a weight matrix into internal stimuli of the neuron associated with the frequency band. In a second analysis step, in the neuron associated with each frequency band, the number of stimuli is determined and then compared with an internal firing threshold. If this threshold is exceeded, the corresponding neuron fires. The firing of the corresponding neuron can then generate a control signal to control one or more actuators.
A preferred embodiment provides that the firing signal of the neuron(s) be applied to a statistics module, which also receives other data as input variables, for example, data regarding exposure time control, so-called clipping detection, expert knowledge, wiper status information, as well as other variables, based on which the actual control of the actuators is then effected. The statistics module can furthermore be used to adapt the weight matrix, adapt the firing conditions, control active infrared illumination, or the like. Such an analysis of the spatial frequency spectra permits precise wiper control that can automatically adapt to changing environmental conditions.
Advantageously, the spatial frequency spectra of an image detected by the optical sensor array are determined by transforming the image data with a discrete cosine transform. This discrete cosine transform is the real component of the two-sided complex Fourier transform, which reduces the computation effort required to realize the transformation.
Between the step of determining the spatial frequency spectra and executing the comparison step, it is advantageous to subject the individual spatial frequency bands to time-recursive lowpass filtering or to spatially discrete time-recursive lowpass filtering. Such filtering suppresses the analysis of phantom frequencies.
Since a robust exposure time control for operation in daylight conditions can follow a rapid change only with a certain dead time, the sensor signal may overload for short periods at individual points of the signal. It is therefore advantageous to supply the individual pixels of the optical sensor array to an overload determination, which determines whether individual pixels are overloaded with respect to their detection range. The signal of an overloaded pixel is distinguished in that the rising signal edge suddenly changes to the horizontal in a sharp bend. The other signal edge is formed accordingly. These two bends in the signal when transformed into the spatial frequency range result in supposed high-frequency image components that do not actually exist. If an overload of one or more pixels is determined, this is taken into account when the comparison step is executed. It is advantageous if such an overload-monitoring unit correspondingly influences the evaluation unit so that the generation of stimuli by overloaded signal components is inhibited.
Even though it is not necessary in principle for the implementation of the process as intended additionally to illuminate the windshield, there may be driving situations where the residual ambient light is not sufficient to detect the objects adhering to the windshield. For this purpose, such a device may be associated with an illumination unit that illuminates the windshield from the inside. This illumination unit emits infrared light such that said light strikes the windshield at alternating angles of incidence. The light reflexes generated by the illumination are used as the basis to determine the spatial frequency spectrum in the detected image segment.
When a camera sensor is used as the sensor array, it can furthermore be used as a detector module for other image processing techniques.